The present invention relates to an exposure control apparatus for a camera, and more particularly to an exposure control apparatus capable of obtaining a correct exposure of a main subject even under a rear lighting or spot illumination.
There is known a method of obtaining a correct exposure suitable for any subject brightness, for example, as disclosed in Japanese Patent Publ. No. 56-18933, Japanese Patent Laid-Open Publ. No. 57-129424 or Japanese Patent Laid-Open Publ. No. 57-38419. According to this method, as the shutter blade is opened, a light-receiving aperture disposed in front of a light-receiving element is opened to measure the available light, and the shutter blade is closed in response to a shutter close signal which is generated when the integrated value of a photocurrent from the light-receiving element reaches a predetermined value. With this exposure control method, photometry is carried out in parallel with the opening of the shutter blade, so that a correct exposure control without time delay can be ensured, and in addition, the mechanism is not complicated.
With the above-described exposure control method, however, the light-receiving element measures the average brightness of the whole field of view, and the reference value of the integrated photocurrent for obtaining the shutter close signal is determined such that the overall density of a photograph can be reproduced at 18% grey level. If there is a great difference in brightness between the main subject and background within the field of view, as in the case of a subject under rear lighting or spot illumination, there arises the problem that an exposure time suitable for a correct exposure of the main subject cannot be obtained.
There is also known a multi-pattern photometry method wherein photometry is carried out for a plurality of regions within a field of view to calculate the exposure time in accordance with a plurality of measured light values. According to this multi-pattern photometry method, an exposure time calculated from a plurality of measured light values is corrected in accordance with a scene condition such as front lighting, rear lighting, spot illumination or the like, the exposure then being determined in accordance with the brightness distribution within the field of view. As a result, if the shutter is opened and closed in accordance with the calculated exposure time, the main subject can be automatically and correctly exposed.
The conventional exposure control method using multi-pattern photometry can obtain a correct photometry and exposure calculation. However, the shutter opening/closing control itself is not monitored at all so that it is impossible to confirm whether the exposure has been carried out or not for the calculated exposure time. As a result, if the shutter opening/closing control is subject to error, this error cannot be corrected thereby resulting in an incorrect exposure.